Bonding of lead wires between a microcircuit chip and the lead frame on which the chip is mounted for coupling to external circuitry is generally accomplished by "ball/wedge" bonding. As shown in FIG. 1, a ball 15 of lead wire metal is formed at the end of the lead wire or bonding wire 11 by melting and solidifying the end of the lead wire. Such ball formation is accomplished, for example, by flame torch or electrical discharge as further described in the U.S. patent applications cross-referenced above. After solidification, the metal ball 15 at the end of the wire is brought into intimate contact with the metalized die pad 14 as shown in FIG. 2. A ball bond is formed typically by thermocompression bonding, applying a specified force and temperature for a specified period of time. Metallic welding and diffusion combine to form this basic ball bond. Alternatively, ultrasonic bonding or other form of energy may be used.
The capillary tool 12 is then raised to a specified level above the ball bond and chip with the lead wire feeding through the capillary tool. The capillary tool 12 and chip or other substrate are then moved relative to each other for bonding of the lead wire at another location such as, for example, a lead frame finger 18. Such relative motion is typically accomplished in ball bonding machines by translating the lead frame strip beneath the raised capillary tool. At this new location the lead wire 11 is brought into intimate contact with the surface of lead frame finger 18 to form a so-called "wedge bond" or "weld". The wedge bond is formed by the side tip of capillary tool 12 bearing down on the lead wire 11 against the surface of lead frame finger 18 as shown in FIG. 4. The lead wire 11 is then severed above the wedge bond 17 by clamping the lead wire 11 at a clamp not shown and raising the bonding tool 12 and lead wire 11 above the wedge bond 17 as shown in FIG. 5. In this manner, a sound lead wire connection is established between the metalized substrate of a chip and the lead frame for coupling to external circuitry. Further background on ball bonding and wedge bonding of lead wires can be found in the article "Evaluating Wire Bond Quality" by Stephen W. Hinch and Donald R. Cropper in the February 1981 issue of Semi Conductor International, and the U.S. patent applications cross-referenced above.
During the wire bonding operation, the failure of the ball 15 to bond securely and adhere to the die pad 14 results in a missed ball bond referred to herein as a "bond attempt". During normal operation when the ball 15 on the end of the lead wire or bonding wire is pushed or compressed against the die pad in the presence of heat and ultrasonic energy, the ball 15 is flattened and good bonding or adhesion is obtained between the ball 15 and die pad 14 as shown in FIG. 3. When the capillary tool 12 is raised by the bonding machine the bonding wire 11 is pulled or drawn through the capillary tool 12 by the good adhesion between ball 15 and die pad 14. On the other hand, if only poor adhesion or insufficient bonding is obtained between the ball 15 and die pad 14, the flattened ball is lifted off the chip when the capillary tool 12 is raised. Thus, even though the lead wire 11 is free to feed through the tool 12, frictional forces and forces affected by withdrawal of the tool 12 are usually sufficient to lift off the flattened ball 15A in the event of poor bonding as shown in FIG. 6. In any event, the lateral movement of the lead frame strip relative to capillary tool 12 will remove a poorly bonded ball from the die pad 14. The failure of a ball 15 to adhere to the die pad 14 is referred to herein as "missed bond" or a "bond attempt". In each instance of a bond attempt, the flattened ball 15A is parted from the die pad 14 upon raising of the capillary tool or translation of the lead frame strip and bonding tool relative to each other. Such occurrence of a bond attempt necessitates corrective action at the ball bonding machine. For example, the operator of the manual type ball bonding machines, such as the High Speed Tailless Thermocompression Ball Bonder, Model 478, of Kulick & Soffa Industries, Inc., must move the capillary tool and "weld off" the flattened ball 15A at a non-interfering or out of the way location on the lead frame strip such as the paddle support. Thus, the missed ball bond is wedge bonded or welded to the paddle support and the lead wire is severed by raising of the tool and lead wire for formation of a new ball bond. The new ball is then rebonded to the die pad and the bonding operation is resumed.
The occurrence of bond attempts or missed ball bonds becomes even more critical for the new automated ball bonding machines and robots such as the Kulick & Soffa Industries, Inc., Model No. 1419, Hi Speed Ball Bonder. With many lead wire bonds and welds formed per minute, a missed ball bond must be detected immediately if any corrective action is to be taken.
Similarly, a wedge bond or weld may fail to adhere to the lead frame finger 18. Under normal operating conditions, metallic welding and diffusion combine to form a sound wedge bond with adhesion to the lead frame finger as shown in FIG. 5. In the event of insufficient bonding and poor adhesion, however, the wedge bond will part from the lead frame finger when the capillary tool 12 and lead wire 11 are raised from the surface of the lead frame finger as shown in FIG. 7. Such failure of the wedge bond to adhere to the lead frame finger surface is referred to herein as "missed wedge bond", "weld attempt" or "wedge bond attempt". After a weld attempt the lead wire passing through capillary tool 12 remains connected with the ball bond at the die pad 14 of chip 16 though weakened in the location 19 of the weld attempt. Such a wedge bond attempt or weld attempt may necessitate rejection of the microcircuit chip and it is advantageous to detect such an attempt upon occurrence for marking the corresponding microcircuit chip. In the event of such a wedge bond attempt or weld attempt, the lead wire is severed at the weakened location 19 of the wedge bond attempt by further relative motion of the lead frame strip and bonding tool relative to each other. During such corrective action, the lead wire 11 remains clamped for movement with the bonding tool 12 by a clamp not shown but part of the equipment of the standard ball bonding machine. Immediate detection of weld attempts or wedge bond attempts is further important in order to inhibit the ball bonding machine from proceeding to the ball formation operating mode before the lead wire is severed at the weakened location of the missed wedge bond.
Conventional ball bonding machines, whether partially manual or automated, generally include a number of operating modes. During the ball formation operating mode or "flame off", a ball is formed at the end of the lead wire by, for example, electrical discharge between a high voltage source and the lead wire. During such ball formation operating mode the lead wire must be grounded. Upon completion of the ball formation or ball formation operating mode, the bonding machine implements a ball bonding operating mode during which the ball is bonded to a die pad of an integrated circuit chip. Following the ball bonding operating mode the bonding machine implements a wedge bonding operating mode for bonding or welding the other end of the lead wire to the lead frame finger. Upon severing of the lead wire, the bonding machine is ready to return to the ball formation operating mode. The progress of the bonding operation from one operating mode to the next is, of course, dependent upon the formation of good quality bonds with strong adhesion. In the event of bond attempts and weld attempts, it is advantageous, of course, to implement corrective action before the bonding machine progresses to the next operating mode.